1. Field of Use
This invention relates generally to a method for operating a dual slide valve rotary gas compressor to prevent undesirable compression of oil in the gas compression chambers during compressor start-up.
2. Description of the Prior Art
Rotary gas compressors are used, for example, in refrigeration systems to compress refrigerant gas, such as "Freon", ammonia or the like. One new type of rotary gas compressor employs a housing in which a motor-driven single main rotor having spiral grooves thereon meshes with a pair of gate or star rotors on opposite sides of the rotor to define gas compression chambers. The housing is provided with two gas suction ports (one near each gate rotor) and with two gas discharge ports (one near each gate rotor). Two dual slide valve assemblies are provided on the housing (one assembly near each gate rotor) and each slide valve assembly comprises a suction slide valve and a discharge slide valve for controlling an associated suction port and an associated discharge port, respectively. During operation of the compressor, a small amount of oil is continuously supplied to the compression chambers to provide an oil seal at points where the main rotor meshes with the gate rotors and with the housing to thereby effectively seal the chambers against gas leakage during gas compression. The oil flows out through the discharge ports and is recovered and recirculated. When the compressor is shut down and coasting to rest, excess oil can collect or settle in the compression chambers. When the compressor is restarted, the residual oil in the compression chambers, plus fresh oil entering the compression chambers, must be expelled through the discharge ports.
U.S. Pat. Nos. 4,610,612 and 4,610,613, both issued on Sept. 9, 1986, and both assigned to the same assignee as the present application, disclose the aforedescribed new type of dual-slide valve rotary gas compressor and control means for operating the slide valves.
The electric motors employed to drive rotors in rotary compressors are usually of a type which requires the compressor to be unloaded while being started and brought up to some predetermined normal constant speed. Loading and unloading is accomplished by positioning of slide valves which control admission and discharge of gas into and from the compression chambers.
Some prior art rotary compressors employ a movable single slide valve to control both the suction port and the discharge port simultaneously. Unloading of such a compressor for startup requires that the single slide valve be moved to unloaded position wherein the suction port is fully open and the discharge port is fully closed, except for a small fixed discharge port. Under these port conditions, very little gas compression occurs. However, such closure of the discharge port would interfere with the exit flow of oil in the compression chambers which is being driven therethrough toward the discharge port during start-up. The compressor tries to compress an incompressible fluid (oil), and the hydraulic pressure build-up can be great enough to cause damage to compressor components. The remedies for this are to drain residual oil before start-up to prevent serious hydraulic pressure build-up upon start-up and avoid damage to compressor components. Draining oil before start-up is not a simple task because, in a pressure-equalized compressor system, a gravity drain system must be used. If there is no drain reservoir at a lower elevation than the compressor, substantial design modifications must be made. On the other hand, since the amount of oil present at start-up can vary and cannot be accurately determined, designing or operating a single slide valve to provide an oil flow passage through the discharge port, which is large enough or always open far enough to accommodate oil flow during start-up, is practically impossible and would adversely affect compressor efficiency.
The initial approach used by the present applicant to operate the new type dual slide valve rotary gas compressor and controls disclosed in the aforementioned U.S. Pat. Nos. 4,610,612 and 4,610,613, was based on the teachings of and experience with prior art single slide valve rotary compressors. The prior art teaching was to effect start-up of a compressor while it was fully unloaded, i.e., with the suction port fully open and the discharge port fully closed (except for a relatively small fixed discharge port). Application of the prior art teaching to the new dual slide valve compressor led applicant to dispose the independently movable suction slide valve in fully unloaded position and to dispose the independently movable discharge slide valve in corresponding fully unloaded position, i.e., nominally fully closed, but with a relatively small fixed discharge port, as in prior art single slide valve rotary compressors. Therefore, the control means for the dual slide valves were, designed, constructed, interconnected and operated to achieve this result and performance was generally satisfactory. However, under certain unpredictable operating conditions during start-up, as when a large amount of residual oil accumulates and cannot exit rapidly enough through the oil exit passage, there is rapid oil pressure build-up which is sufficiently high to cause component damage in the compressor. This has occurred, even though the end of the discharge slide valve that cooperates with the discharge port, and the discharge port itself, were designed, constructed and sized in accordance with prior art teachings to provide a passage believed to be of sufficient size to allow for the unrestricted exit of oil when the discharge slide valve was in nominally fully closed position, i.e., conventional fully unloaded position.
Efforts aimed at overcoming this serious problem involved several approaches. First, consideration was given to redesign of the compressor to provide an oil drainage system to entirely eliminate the possibility of oil pressure build-up on start-up. This solution is costly and not available for all compressor installations. Second, consideration was given to redesign of the discharge slide valve and discharge port to provide a larger oil exit passage while the discharge slide valve was nominally closed to mitigate the likelihood of oil pressure build-up during start-up. This solution is costly, still unreliable and introduces problems of inefficiency. Finally, applicant conceived the idea of operating the dual slide valves in a novel and unobvious manner which differed from that initially employed and of adapting the control means to effect such operation by rearranging and changing the sequence of operation of the control means. This method of operation proved to be entirely workable and satisfactory, overcame oil pressure build-up during compressor starting, eliminated the risk of component damage, and involved minimum costs. This method is the subject of the present invention.